After a set number of quests or after some instances, you may be able to see the Hero Treasure Chest interface. Each Hero Treasure Chest can be opened for items three times -- the first time free, the second and third require gold.
How to Open Edit
You will be able to open a Hero Treasure Chest after completing 20 Daily Tasks, 20 Daily Rewards Quests, 3 Daily Escorts, 20 Daily Treasure Quests, 20 Daily Demonslaying Quests, 10 Daily Guild Quests or 100 Weekly Tasks (over the week).
You may also be able to open a Hero Treasure Chest after instances such as Experience Trials, Money Vault, Material Groove (Basic, Intermediate or Advanced), Costume Corner, Combo King, Pet Place, Feather Zone, Goddess Defence (Normal, Heroic or Nightmare), Icy Impass (Normal, Heroic or Nightmare) or the multiplayer instance, Romantic Interlude. However, this is based on chance and you may not always be able to open a Chest even after completing all the instances.
This means that the maximum number of chests that can be opened for free per day is, if you're lucky, is 23.
There are sometimes rare items, such as Lvl. 5 ATK Gem Pack, 999 Flowers, Lvl. 4 Carving Token etc and they may show up in the announcements when someone is lucky enough to receive one. Most items, however, are Basic Refinement Stone, Lvl 1 Carving Token or Basic Refinement Scroll. This is based on luck.
You can see the prizes you and other people have gotten in the "Adventure Record" on the left side of the interface.
You may also receive 5 achievement points after opening 1k Hero Treasure Chests. This is approximately 25 vouchers and a 250 max HP increase, which means might increases.